For those pups who love to pull and lunge, the Gentle Leader sets them straight by redirecting their head from where they want to go back toward you. As you can imagine, this infuriates some canines and makes them eager to paw at the pesky collar in attempt to remove it. Half the battle of teaching your pup proper Gentle Leader manners is reinforcing good behavior.
Loosen or tighten the collar as needed. If the Gentle Leader doesn't fit properly, it's going to cause problems with how it affects your pup on his walks. You should be able to fit no more and no less than one finger between the strap that goes around the top of his neck. For the strap that goes around your pup's snout, see how far you can pull it down. If it just touches the leathery portion of his nose, you're good to go. If it slides down any farther, tighten it up a bit.
Reward him for wearing it. At first, your pup will hate the Gentle Leader. And as you probably know, when he hates something, everything becomes more difficult. So bring in some treats to show him that wearing his special collar is exciting, or at least that it leads to tasty snacks. Immediately after you attach the collar, give him a treat and praise him. Take off the Gentle Leader and repeat the process over and over again, leaving it on longer before you toss him his reward. When he's wearing it without freaking out for about a minute, you're ready to move on.
Use a standard 4- or 6-foot leash or keep a retractable leash locked in position. Walk with your hand at your side and keep it there as your pup tries to pull. Take a couple of steps and tell your pup to sit, then reward him with a tasty treat. Do this for the first five minutes so your pup realizes that his new collar contraption isn't such a bad idea.
Distract him when he tries to remove it. Despite all those treats in his tummy and all the positive associations he's had with the Gentle Leader, your pup is almost certain to try and remove it at some point. Sometimes he'll come to a dead stop while walking and bat his face with his paws, or he might try to walk and remove it at the same time. Say "ah" sharply to distract him. Once he stops, hand him a treat and give him a bit of praise.
Slowly come to a stop if he pulls excessively. If your pup's trying to jerk his head forward and push ahead, slow down to a stop -- never abruptly -- and tell him to sit. Some dogs get so frustrated with the collar that they'll stubbornly try to press on at times. Stopping resets his behavior and calms him down.